Fiber coupler with an optical window

ABSTRACT

A fiber array unit (FAU) includes a substrate, a plurality of optical fibers, and a lid. The substrate includes: an optical window extending through a layer of non-transparent material, a plurality of grooves, and an alignment protrusion configured to mate with an alignment receiver. The plurality of optical fibers are disposed in the plurality of grooves. The alignment protrusion is configured to align the plurality of optical fibers with an external device when mated with the alignment receiver. The plurality of optical fibers is disposed between the lid and the substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a divisional of co-pending U.S. patent applicationSer. No. 15/946,930, filed Apr. 6, 2018. The aforementioned relatedpatent application is herein incorporated by reference in its entirety

TECHNICAL FIELD

Embodiments presented in this disclosure generally relate to photonicdevices, and more specifically, to edge coupling with photonic devices.

BACKGROUND

Generally, photonic chips have interfaces to permit optical signals tobe received from an optical source (e.g., a laser or an optical fiber)or transmitted to an optical fiber. One such method is edge couplingwhere the optical fiber is coupled to the edge of the photonic chip. Asthe level of integration, speed of operation, and functionalityincrease, photonic chips are running out of peripheral bond pad space toallow wire bond based interconnection to the underlying substrate orprinted circuit board (PCB). Thus, photonic chips with Through SiliconVias (TSVs) are highly desirable as they allow for higher density ofinterconnects and reduce the resistance as well as inductance associatedwith the wirebond connections. It is also desirable to develop lowercost and more efficient approaches to attach optical components, such asfibers and lasers, to these TSV-compatible photonics chips.

BRIEF DESCRIPTION OF THE DRAWINGS

So that the manner in which the above-recited features of the presentdisclosure can be understood in detail, a more particular description ofthe disclosure, briefly summarized above, may be had by reference toembodiments, some of which are illustrated in the appended drawings. Itis to be noted, however, that the appended drawings illustrate onlytypical embodiments of this disclosure and are therefore not to beconsidered limiting of its scope, for the disclosure may admit to otherequally effective embodiments.

FIG. 1 illustrates an optical system, according to one embodimentdisclosed herein.

FIG. 2 illustrates alignment features for coupling a photonic chip to afiber array unit, according to one embodiment disclosed herein.

FIG. 3 illustrates coupling a fiber array unit to the photonic chip,according to one embodiment disclosed herein.

FIG. 4 illustrates a fiber array unit with an optical window, accordingto one embodiment disclosed herein

FIG. 5 illustrates a photonic chip with epoxy wells and alignmentreceivers, according to embodiments disclosed herein.

FIG. 6 illustrates coupling a fiber array unit to a photonic chip,according to one embodiment disclosed herein.

FIG. 7 illustrates coupling a fiber array unit to a photonic chip,according to embodiments disclosed herein.

FIG. 8 illustrates mating an alignment protrusion with an alignmentreceiver, according to one embodiment disclosed herein.

FIGS. 9A-9C illustrate mating a misaligned alignment protrusion with analignment receiver, according to embodiments disclosed herein.

FIG. 10 illustrates mating an alignment protrusion with an alignmentreceiver, according to embodiments disclosed herein.

FIG. 11 illustrates mating an alignment protrusion with an alignmentreceiver, according to one embodiment disclosed herein.

FIG. 12 illustrates mating an alignment protrusion with an alignmentreceiver, according to one embodiment disclosed herein.

FIGS. 13A-13C illustrates forming alignment protrusions and an opticalwindow for the fiber array unit in FIG. 4 , according to one embodimentdisclosed herein.

FIGS. 14A and 14B illustrate a fiber array unit with a siliconsubstrate, according to one embodiment disclosed herein.

FIGS. 15A-15C illustrate forming alignment protrusions and an opticalwindow in the fiber array unit in FIGS. 14A and 14B, according to oneembodiment disclosed herein.

FIGS. 16A and 16B illustrate a fiber array unit with a siliconsubstrate, according to one embodiment disclosed herein.

FIGS. 17A and 17B illustrate forming alignment protrusions and anoptical window in the fiber array unit in FIGS. 16A and 16B, accordingto one embodiment disclosed herein.

FIG. 18 illustrates a molded part for a fiber array unit, according toone embodiment disclosed herein.

To facilitate understanding, identical reference numerals have beenused, where possible, to designate identical elements that are common tothe figures. It is contemplated that elements disclosed in oneembodiment may be beneficially utilized on other embodiments withoutspecific recitation.

DESCRIPTION OF EXAMPLE EMBODIMENTS

Overview

One embodiment described herein is a fiber array unit (FAU) thatincludes a substrate that has an optical window extending through alayer of non-transparent material, a plurality of grooves, and analignment protrusion configured to mate with an alignment receiver. TheFAU also includes a plurality of optical fibers disposed in theplurality of grooves where the alignment protrusion is configured toalign the plurality of optical fibers with an external device when matedwith the alignment receiver and a lid where the plurality of opticalfibers is disposed between the lid and the substrate.

Another embodiment described herein is an apparatus that includes asemiconductor substrate, a plurality of optical fibers, and a lid. Thesemiconductor substrate includes an optical window extending through alayer of non-transparent material of the semiconductor substrate, aplurality of grooves, and an alignment protrusion configured to matewith an alignment receiver. The plurality of optical fibers are disposedin the plurality of grooves. The alignment protrusion is configured toalign the plurality of optical fibers with an external device when matedwith the alignment receiver. The plurality of optical fibers is disposedbetween the lid and the substrate.

Example Embodiments

Generally, photonic chips have an optical interface to transmit opticalsignals to an optical fiber, or to receive optical signals from anoptical source such as a laser or optical fiber. Some optical interfacesinclude edge couplers disposed at the sides of the photonic chip, whichmakes the photonic chips easier to manufacturer and improve opticalcoupling compared to other solutions. However, photonic chips with TSVshave several additional constraints on edge coupling. Wafers with TSVsare thinner (typically in the range of 50 um to 150 um). Hence, eventhough shallow trenches in a silicon substrate are possible, deeptrenches (typically created by Deep Reactive Ion Etching (DRIE)) cannotbe created to accommodate a lens or fiber placement for an edge coupler.In addition, TSVs constrain the overall optical packaging or assemblysince photonic chips with TSVs are typically attached to a glass orsilicon interposer or a ceramic or an organic substrate usingconventional solder reflow or thermal compression bonding processes. Assuch, conventional edge coupling techniques cannot be used with aphotonic chip that has TSVs. However, the embodiments described hereincan also be applied to other coupling approaches, such as evanescentcoupling and vertical coupling from the die surface.

In order for optical components (e.g., photonic chip, optical fiber,laser, etc.) to function properly, the optical components are alignedwith each other. Optical alignment is the process of aligning opticalelements with one another to maximize the accuracy and performance oftransmitted signals. Active alignment involves a person (or aligningmachine) viewing and aligning the different components based on feedbackwhen transmitting optical signals between the components, whichincreases the cost of manufacturing photonics chips. Passive alignment(also referred to as mechanical alignment) relies on strictmanufacturing tolerance of components and optical based initialplacement to ensure the components are aligned properly when thecomponents are placed at their respective position without aligning thecomponents based on feedback—i.e., without transmitting optical signalsbetween the components.

Embodiments herein describe a fiber array unit (FAU) configured to aligna plurality of optical fibers to a photonic chip. The FAU has aplurality of grooves for receiving the plurality of optical fibers. Inone embodiment, the FAU includes at least one alignment feature thatcorrespond to an alignment feature in the photonic chip to achievepassive alignment. A die bonder may be used to place the FAU on thephotonic chip using the alignment features for precise alignment.Instead of using alignment features that may align using a slidingmotion, for example, by sliding the FAU along a side of the photonicchip, the embodiments herein use alignment features which permit a diebonder to align the FAU to the photonic chip by pressing a top of thephotonic chip into contact with a bottom surface of the FAU. In oneembodiment, the alignment features in the FAU include one or morefrustums while the alignment features in the photonic chip includecorresponding square or rectangular trenches.

Epoxy (or other adhesive) can be used to bond the FAU to the photonicchip when the optical fibers attached to the FAU are aligned with anoptical interface in the photonic chip. To facilitate curing the epoxybetween the FAU and the photonic chip, embodiment of the FAU includesone or more optical windows that overlap epoxy dispensed on the photonicchip which permits radiation (e.g., UV light or microwave radiation) topass therethrough. As such, during curing, UV light can pass through theoptical windows in the FAU to cure the epoxy disposed between the FAUand the photonic chip.

FIG. 1 illustrates an optical system 100, according to one embodimentherein. As shown, the optical system 100 has a substrate 105 and aninterposer layer 110 connected via solder 115. The interposer layer 110is a layer with through electrical connections and routing layers onSilicon, Glass, Ceramic or organic material. The interposer layer 110 iscoupled to a redistribution layer (RDL) 120. Coupled to the RDL 120 arean application specific integrated circuit (ASIC) 130, a high bandwidthmemory (HBM) 135, and a photonic chip 140 which includes a semiconductormaterial. The RDL 120 allows electrical connections to be made betweenelectrical components coupled to it. Stated differently, the RDL 120allows components (e.g., the ASIC 130, the HBM 135, the photonic chip140, etc.) to communicate electrically by minimizing external electricalconnections. As shown, the interposer layer 110 has a plurality ofThrough Silicon Vias (TSVs) 125, which couple the RDL 120 to thesemiconductor substrate 105. While the interposer layer 110 is shownwith TSVs, the interposer layer 110 may be made of glass in which casethe interposer layer 110 would be a through via or a through oxide via.In one embodiment, the TSVs 125 provide power to the RDL 120 which inturn routes the power to the ASIC 130, the HBM 135, and the photonicchip 140.

As shown, the photonic chip 140 is coupled to a driver 145 and a FAU150. The driver 145 sends/receives signals to/from an optical fiber 155via the FAU 150 and the photonic chip 140. In another embodiment, thedriver 145 is a transimpedance amplifier that amplifies the electricalsignals generated by an optical detector (not shown) in the photonicchip 140 in response to photonic signals received from the optical fiber155 mounted on the FAU 150. As shown, the photonic chip 140 has aplurality of TSVs 160. In one embodiment, the photonic chip 140 providespower from the Printed Circuit Board (PCB) or organic/ceramic substratethrough the interposer layer 110 to the driver 145 via one of the TSVs160.

In one embodiment, the ASIC 130 and the driver 145 communicate via theTSVs 160 in the photonic chip 140, as well as the interposer layer 110and the RDL 120. In one embodiment, the ASIC 130 includes logic forproviding data to and from the photonic chip 140 from outside the system100. For example, the ASIC 130 can send signals to the driver 145 suchthat the driver 145 sends a signal to a modulator (not shown) in thephotonic chip 140, and the modulator encodes the data from the driver145 onto an optical signal. In one embodiment, at high speed operation,the driver 145 is placed directly onto the photonic chip 140 to provideelectrical connections that are as short as possible. In one embodiment,the optical detector in the photonic chip 140 outputs voltages based ona received optical signal to the driver 145. The driver 145 in turnprovides data to the ASIC 130 based on the received signal. In oneembodiment, the HBM 135 stores settings for the ASIC 130 which dictatehow the ASIC 130 communicates between the driver 145 and externaldevices and systems. In another embodiment, the HBM 135 stores settingsfor how the photonic chip 140 receives and transmits optical signals.

In one embodiment, the photonic chip 140 is a photonics transceiver thatreceives and transmits optical signals. For example, an optical signalmay be transmitted along the optical fiber 155 where the photonic chip140 receives the optical signal. As another example, the photonic chip140 transmits an optical signal to the optical fiber 155. In thismanner, the photonic chip 140 can communicate using the optical fiber155 to an external system. In one embodiment, the photonic chip 140 isan optical modulator that is controlled by electrical data signalsreceived from the driver 145. In another embodiment, the photonic chip140 is an optical detector that transmits electrical signals to the ASIC130 via the driver 145. Specifically, the TSVs 160 of the photonic chip140 and traces on the PCB or organic/ceramic substrate have anelectrical signal that corresponds to an optical signal detected by thephotonic chip 140. In this manner, the optical system 100 may sendand/or receive optical signals.

FIG. 2 illustrates alignment features for coupling the photonic chip 140to a FAU, according to one embodiment disclosed herein. As shown, thefeatures are formed in a top surface 220 of the photonic chip 140 onwhich the driver 145 is mounted. In this embodiment, the featuresinclude epoxy wells 205, alignment receivers 210, and an opticalinterface 215. The epoxy wells 205 may include etched portions of thetop surface 220 that have been recessed for receiving epoxy (or otheradhesive that can be cured using UV radiation) for coupling the photonicchip 140 to the FAU (not shown). In this example, the two epoxy wells205 include raised features (e.g., islands). The raised features mayhave the same height as the other portions of the top surface 220 of thephotonic chip 140. However, the area of the epoxy wells 205 surroundingthe raised features is recessed relative to the top surface 220 to forma containment area for the epoxy.

In one embodiment, the photonic chip 140 also includes epoxy wellsdisposed near the optical interface 215. These epoxy wells may, or maynot, have raised features like the ones shown in the epoxy wells 205. Inone embodiment, all the epoxy wells 205 on the photonic chip 140 havethe same depth and are formed during the same etching process.

The alignment receivers 210 are designed to receive correspondingalignment features in the FAU. The arrangement of the receivers 210 inthe photonic chip 140 may enable passive alignment in at least onealignment direction. For example, by aligning the alignment features inthe FAU to the alignment receivers 210, the optical fibers mounted tothe FAU are aligned with the optical interface 215 in at least one ofthe X, Y, or Z directions such that optical signals can be transferredbetween the photonic chip 140 and the optical fibers via the opticalinterface 215. The alignment receivers 210 may have the same depth asthe epoxy wells 205 or a different depth. In one embodiment, thealignment receivers 210 are rectangular or square trenches formed in thetop surface 220.

In one embodiment, the photonic chip 140 includes one or more TSVs, andthus, its thickness may be limited as explained above. However, theembodiments herein are not limited to edge coupling an FAU to a photonicchip 140 with TSVs but can be used in a photonic chip that does notincludes TSVs.

FIG. 3 illustrates coupling the FAU 150 to the photonic chip 140,according to one embodiment disclosed herein. As shown, the FAU 150includes the optical fibers 155 which are mounted between a substrate305 and a lid 310. In FIG. 3 , the substrate 305 is shown as beingtransparent so that the underlying details of the FAU 150 and thephotonic chip 140 can be seen. In some embodiments, the substrate 305may be made from a transparent material (such as glass) ornon-transparent material (such as crystalline silicon). As used herein,“transparent” when used in context of the substrate 305 refers to amaterial that permits electromagnetic radiation that can cure epoxy topass therethrough. Put differently, when formed from a transparentmaterial, the substrate 305 can be formed from any material which istransmissive (or transparent) to radiation used to cure the epoxydisposed in the epoxy wells 205. In one embodiment, the substrate 305 istransparent to ultra violet (UV) light 315 which is used to cure theepoxy. Suitable materials for the transparent substrate 305 could beglass or silicon dioxide.

However, in FIG. 3 it is assumed that at least a portion of thesubstrate 305 is made from a non-transparent or opaque material. Thatis, the UV light 315 may unable to pass through the substrate 305 toreach the epoxy wells or the optical interface 215 where epoxy may bedisposed. To aid in curing the epoxy, the substrate 305 includes anoptical window 325 through the substrate 305. The optical window 325 canbe an etched feature in the substrate 305 or be formed in the substrate305 when the substrate is fabricated (e.g., if the substrate 305 is amolded part). The advantage of a non-transparent substrate 305 is thatit can be formed from more cost effective materials and less complicatedprocesses relative to a transparent substrate. For example, instead ofusing a transparent substrate, the substrate 305 can be formed entirelyfrom silicon (as shown in FIGS. 14A-B and 16A-B) which is less expensiveand may use less complicated fabrication processes. Although at leastpartially formed from a non-transparent material, adding the windows 325in the substrate 305 still permits the UV light 315 (or other types ofradiation such as microwave radiation) to reach and cure the epoxydisposed between the FAU 150 and the photonic chip 140.

In one embodiment, the optical fibers 155 are aligned to respectivewaveguide adapters (not shown) at the optical interface 215. That is,the waveguide adapters can be exposed at the optical interface 215 (orrecessed slightly away from the optical interface 215—e.g., a fewmicrons) so that light can be transferred between the optical fibers 155and waveguides in the photonic chip 140. In one embodiment, the FAU 150is passively aligned to the photonic chip 140 using the alignmentreceivers 210 and corresponding alignment features in the FAU 150 (whichare not shown in FIG. 3 ). That is, using a die bonder (or otheralignment means), the FAU 150 can be lowered in the directionperpendicular to the top surface 220 such that alignment features in theFAU 150 mate with the alignment receivers 210 in the photonic chip 140with sufficient precisions so that active alignment is not needed.

FIG. 4 illustrates a FAU 150 with an optical window 420, according toone embodiment disclosed herein. Specifically, FIG. 4 illustrates a viewof a bottom surface of the substrate 305 in the FAU 150 which is in afacing relationship with the photonic chip. In this embodiment, thesubstrate 305 includes both transparent and opaque materials. Atransparent layer 410 is disposed above a silicon layer 405. Thetransparent layer 410 can include silicon dioxide that forms a Salstructure with the silicon layer 405. In another embodiment, thetransparent layer 410 is glass or other transparent material whichpermits UV light for curing epoxy to pass therethrough.

The silicon layer 405 is processed to include alignment protrusions 415which each align to a corresponding one of the alignment receivers 210illustrated in FIG. 2 . In this example, the alignment protrusions 415are frustums that extend or protrude from the silicon layer 405 suchthat the protrusions 415 can mate with the alignment receivers 210 inthe photonic chip which passively align the optical fibers 155 to theoptical interface of the photonic chip in at least one alignmentdirection.

The silicon layer 405 also includes an etched optical window 420 whichpermits optical radiation to pass through the silicon layer 405 to reachepoxy disposed between the substrate 305 and the photonic chip. That is,while the transparent layer 410 permits radiation to pass therethrough,the silicon layer 405 does not. Thus, adding the window 420 to thesilicon layer 405 permits light emitting from the top surface of thesubstrate 305 to reach the bottom surface of the substrate 305 and theunderlying epoxy.

FIG. 5 illustrates a photonic chip 140 with epoxy wells 205 andalignment receivers 210, according to embodiments disclosed herein. Inthis example, the epoxy wells 205 and the alignment receivers 210 areformed in an interlayer dielectric (ILD) 505. The ILD 505 can includeoptical couplers which serve as interfaces between waveguides in thephotonic chip 140 and the optical fibers 155 in the FAU 150. In oneembodiment, the thickness or height of the ILD 505 is less than 15microns.

Four alignment receivers 210 are formed in the ILD 505 which mate withthe four alignment protrusions 415 in the FAU 150. That is, the FAU 150and the photonic chip 140 can be moved in a vertical direction that isperpendicular to the bottom surface of the silicon layer 405 and the topsurface of the ILD 505 such that the protrusions 415 interlock with thereceivers 210. Thus, the machine or apparatus that aligns the componentsdoes not need to move in a horizontal motion (e.g., in a directionparallel to the bottom surface of the silicon layer 405 and the topsurface of the ILD 505) to align the optical fibers 155 with opticalcouplers in the ILD 505, although there may be some horizontal motion asthe protrusions 415 interlock with the receivers 210. Put differently,using the alignment features illustrated in FIGS. 4 and 5 , the bondingmachine or apparatus may not need to slide the silicon layer 405 in adirection parallel to the top surface of the ILD 505 in order to alignthe optical fibers 155 to the photonic chip 140.

FIG. 6 illustrates coupling the FAU 150 shown in FIG. 4 to the photonicchip 140 shown in FIG. 5 to form an optical system 600, according to oneembodiment disclosed herein. The silicon layer 405 is shown as beingtransparent so that the details of the underlying structures can beseen. As shown, the protrusions in the silicon layer 405 interlock ormate with the alignment receivers 210 which align the optical fibers 155to optical couplers in the ILD 505. Further, the optical window 420permits radiation emitted through the top surface of the transparentlayer 425 to pass through the silicon layer 405 to reach the surface ofthe ILD 505 on which epoxy is disposed. In this embodiment, the window420 may be arranged such that the radiation can reach index matching (ornon-index matching) epoxy disposed between the ends of the opticalfibers 155 facing the ILD 505. That is, to improve optical couplingbetween the optical fibers 155 and the optical couplers in the opticalinterface of ILD 505, epoxy is disposed in a space between the fibers155 and the optical interface. The window 420 may be formed in thesilicon layer 405 to ensure UV light reaches this space in order to curethe epoxy therein. Further, the window 420 may serve as an epoxyreservoir for holding excess epoxy, which may prevent misalignmentbetween the FAU 150 and the photonic chip 140.

FIG. 7 illustrates coupling the FAU 150 to the photonic chip 140,according to embodiments disclosed herein. Specifically, FIG. 7 is aside view of the optical system 600 illustrated in FIG. 6 . Here, thetips or ends of the optical fibers 155 terminate at the opticalinterface 215 in the ILD 505. There can be an air gap (less than 10microns) between the tips of the optical fibers 155 and the opticalinterface 215. The optical window (not shown in FIG. 7 ) in the siliconlayer 405 permits radiation passing through a top surface 705 of thetransparent layer 425 to pass through the silicon layer 405 and reachthe optical interface 215.

Although FIGS. 4, 6, and 7 discuss a single optical window in thesilicon layer 405 for curing epoxy at the optical interface 215, theembodiments herein are not limited to such. For example, instead ofdisposing an optical window over the interface 215, the optical windowmay instead be disposed over the epoxy wells 205. In another embodiment,the silicon layer 405 may include multiple etched windows where oneoptical window 420 is disposed over the optical interface 215 as shown,and another one or more windows are disposed over the epoxy wells 205.

FIG. 8 illustrates mating an alignment protrusion 415 with an alignmentreceiver 210, according to one embodiment disclosed herein.Specifically, FIG. 8 illustrates a cross section of the alignmentprotrusion 415 and the alignment receiver 210. In one embodiment, thesefeatures may form a frustum and a rectangular trench, respectively.

FIG. 8 illustrates a desired target location 805 where a middle of thealignment protrusion 415 aligns with a middle of the alignment slot 210.That is, for optimal alignment, the middle of the alignment protrusion415 contacts the middle of a bottom surface 830 of the alignmentreceiver 210. In this example, the alignment receiver includes a trenchor cutout in the ILD 505 on the top of the photonic chip 140. The ILD505 may be formed on a substrate 815 of the photonic chip 140 which maybe a semiconductor substrate such as crystalline silicon.

In this example, a bottom surface 850 of the alignment protrusion 415 inthe FAU 150 contacts the bottom surface 830 of the alignment receiver210. Moreover, as discussed in more detail below, the alignmentprotrusion 415 includes self-correcting alignment features 820 (e.g.,the slanted sides of the protrusion 415) which contact sides 825 of thealignment receiver 210 for correcting the alignment of the FAU 150 andthe photonic chip 140 when the middles of the protrusion 415 and thereceiver 210 are not aligned.

FIGS. 9A-9C illustrate mating a misaligned alignment protrusion 415 withan alignment receiver 210, according to embodiments disclosed herein.FIG. 9A illustrates a scenario where the middle of the alignmentprotrusion 415 is offset 905 from the desired target location 805. Thedifference between the offset 905 and the target location 805 isillustrated as a misalignment 910. Stated differently, the misalignment910 is the distance between respective middles of the alignmentprotrusion 415 and the alignment receiver 210.

The misalignment 910 can occur because of tolerances corresponding tothe bonding machine or apparatus (e.g., a die bonder) used to place theFAU 150 on the photonic chip 140. For example, the die bonder mayguarantee that the middle of the alignment protrusion 415 is within+/−10 microns from the middle of the alignment receiver 210 (e.g., thedesired target location 805). FIG. 9A illustrates a worst case scenariowhere the misalignment 910 is the maximum tolerance of the bondingmachine.

To compensate for the tolerance or accuracy of the bonding machine, thealignment protrusion 415 is designed such that regardless of themisalignment 910, the self-correcting alignment feature 820 contacts aside 825 of the alignment receiver 210. That is, the width (W) of thealignment protrusion 415 can be controlled such that the flat, bottomsurface 850 of the protrusion 415 falls within the receiver 210, and asa result, at least one of the self-correcting alignment features 820contacts one of the sides 825.

The accuracy of the alignment in FIG. 8 where the bottom surface 850 ofthe protrusion 415 contacts the bottom surface 830 of the receiver 210may depend on the amount of control of the flatness of the bottomsurface 850 on the protrusion 415 and the tolerance on the etch depth ofthe receiver 210 (which can be around +/−0.5 microns for manydielectrics). Moreover, the slope of the self-correcting alignmentfeatures can be tightly controlled using an orientation dependent etchsuch as a KOH etch and the like.

In FIG. 9A, when the die bonder moves the FAU in the vertical directionillustrated by the arrow 912, the bottom surface 850 is between thesides 825A and 825B. Thus, even at maximum misalignment 910, the bottomsurface 830 is within the receiver 210.

As the FAU 150 continues to move in the direction shown by the arrow912, the self-correcting alignment feature 820A contacts the side 825Awhich is illustrated in FIG. 9B. The die bonder continues to applydownward pressure but the resulting contact between the feature 820A andthe side 825A creates a horizontal motion as shown by the arrow 915which moves the middle of the alignment protrusion 415 closer to themiddle of the alignment receiver 210. That is, in one embodiment, thedie bonder does not apply the horizontal motion directly (e.g., the diebonder may apply pressure in the vertical direction) for the FAU 150 tomove horizontally relative to the photonic chip 140 to correct for themisalignment 910. The vertical pressure applied by the die bonder isconverted into the horizontal motion illustrated by the arrow 915 toalign the piece parts.

FIG. 9C illustrates when the die bonder has moved the parts until thebottom surface 850 of the FAU 150 contacts the bottom surface 830 of thealignment receiver 210. The middles of the alignment protrusion 415 andthe alignment receivers 210 may both be aligned at the target location805, although there may be some remaining misalignment due to thetolerances of the fabrication steps using to form the protrusion 415 andthe receivers 210. However, the tolerances for processing the protrusion415 and the receivers 210 may be much smaller or tighter than thetolerances for the die bonder—e.g., within +/−300 nanometers. Forexample, the alignment protrusions 415 may be formed from silicon or amolded material with very tight fabrication tolerances. Similarly, thetechniques for etching the alignment receiver 210 in the ILD 505 canhave much tighter tolerances than the die bonder.

FIG. 10 illustrates mating the alignment protrusion 415 with thealignment receiver 210, according to embodiments disclosed herein.Unlike in FIG. 8 where the bottom surface of the protrusion 415 contactsthe bottom surface of the receiver 210, in this example, there remains agap between the bottom surface 850 of the protrusion 415 and the bottomsurface 830 of the receiver 210. Instead, the thickness of theprotrusion is controlled such that a bottom surface 1005 of the FAU 150at a base of the frustum formed by the alignment protrusion 415 contactsa top surface 1010 of the ILD 505 (or a top surface of the photonic chip140).

Alignment is achieved by the self-correcting alignment features 820contacting the sides 825 of the receiver 210. That is, the width (W) ofthe protrusion can be controlled such that the bottom surface 850 fitsbetween the sides 825 regardless of the misalignment. This aligns thecomponents in the X and Z directions (i.e., the horizontal axis and theaxis into and out of the page). Y direction alignment (i.e., thevertical axis) is achieved by the bottom surface 1005 contacting the topsurface 1010. The accuracy of this alignment technique depends on theaccuracy of the ILD 505 etch depth which can be around +/−1 micron.

In one embodiment, given the tolerances associated with the fabricationsteps forming the protrusion 415 and the receiver 210, at least one ofthe self-correcting alignment features 820 may contact one of the sides825 when aligned, while at least one other of the self-correctingalignment features 820 does not. However, in other embodiments, multiplealignment features 820 may contact respective sides 825 when aligned.

FIG. 11 illustrates mating the alignment protrusion 415 with thealignment receiver 210, according to one embodiment disclosed herein. Inthis example, the width of the alignment protrusion 415 is againcontrolled such that the bottom surface 850 fits inside the sides 825regardless of the misalignment. However, instead of alignment beingachieved when a flat surface of the FAU 150 contacts a flat surface ofthe photonic chip 140, here the FAU 150 is aligned to the photonic chip140 when the self-correcting alignment feature 820 on one side of theprotrusion 415 and the self-correcting alignment feature 820 on theopposite side of the protrusion 415 both contact respective sides 825 ofthe alignment receiver 210. Although FIG. 11 illustrates theself-correcting alignment feature 820A contacting the side 825A and theself-correcting alignment feature 820B contacting the side 825B, threeor four self-correcting alignment features 820 in the protrusion 415 maycontact respective sides 825 of the receiver 210. Contacting twooppositely disposed self-correcting alignment features 820 to two sides825 of the receiver provide alignment in the X and Y direction.Moreover, when a third self-correcting alignment feature 820 (which isdisposed between the two oppositely disposed alignment features)contacts a side 825 of the receiver 210, this can provide alignment inthe Z direction.

FIG. 12 illustrates mating the alignment protrusion 415 with thealignment receiver 210, according to one embodiment disclosed herein.FIG. 12 relies on a similar alignment principle in FIG. 11 where atleast two opposing self-correcting alignment features 820 contactrespective sides 825 of a trench—e.g., a deep alignment receiver 1205.However, instead of forming the receiver solely within the ILD 505, inFIG. 12 , the deep alignment receiver 1205 extends into the substrate815. In one embodiment, the deep alignment receiver 1205 may have adepth greater than 15 microns. Further, the depth of the alignmentreceiver 1205 may permit the protrusion 415 to have a pyramidal shaperather than a frustum shape as shown in FIG. 12 . That is, theself-correcting alignment features 820 may intersect at a point ratherthan forming a flat bottom surface facing the bottom surface of the deepalignment receiver 1205.

One advantage of using the alignment technique illustrated in FIGS. 11and 12 is the spacing between the bottom surface of the FAU 150 and thetop surface of the ILD 505 can be filled with epoxy for bonding the twocomponents together. However, relying on contact between theself-correcting alignment features 820 and the sides 825 of the receiver210, 1205 can cause stress which may increase the likelihood of chippingthe sides 825.

FIGS. 13A-13C illustrates forming alignment protrusions and an opticalwindow for the FAU 150 in FIG. 4 , according to one embodiment disclosedherein. In one embodiment, the process illustrated in FIGS. 13A-13C isperformed before the optical fibers and lid are attached to thesubstrate 305. Further, the cross section of the substrate 305 in FIGS.13A-13C corresponds to the dotted line A-A illustrated in FIG. 4 .

FIG. 13A illustrates a substrate 305 that includes the transparent layer410 and the silicon layer 405. In one embodiment, the transparent layer410 is silicon dioxide and the substrate 305 is part of a SOI structure.For example, the transparent layer 410 and the silicon layer 405 can becombined to form the substrate 305 using a wafer bonding process andgrinding to achieve a desired thickness of the silicon layer 405.However, the embodiments are not limited these materials or fabricationprocesses.

FIG. 13B illustrates etching the silicon layer 405 in the directionshown by arrow 1305 to form two alignment protrusions 415. To achievethe frustum shape with the sloped self-correcting alignment features820, an orientation dependent etch (e.g., a KOH etch) can be used. Theheight or thickness of the protrusions may depend on which alignmenttechnique is used—e.g., one of the alignment techniques illustrated inFIG. 8 or 10-12 . In one embodiment, the thickness of the protrusion maybe around 25 microns.

FIG. 13C illustrates etching the silicon layer 405 to form the window420. As such, radiation emitted up through the transparent layer 410 canpass through the silicon layer 405 via the window 420. Thus, epoxydisposed between the silicon layer 405 and the photonic chip (not shown)can be cured using UV radiation passing through the window 420.

In one embodiment, the window 420 is formed using an orientationdependent etch such as KOH. Using a mask, a sufficient area of thesilicon layer can be etched to form a desired size of the window 420 atan interface between the silicon layer 405 and the transparent layer410. That is, the size of the area in the silicon layer 405 exposed tothe etching agent is dependent on the angle of the etch and the desiredsize of the window 420 at the transparent layer 410. The etching agentmay selectively etch the silicon layer 405 but not etch the material inthe transparent layer 410.

Further, although not shown in FIG. 13C, the same etching step used toform the window 420 can also be used to form V-grooves for the opticalfibers in the silicon layer 405, although these features can be formedin separate etching steps. For example, instead of using an orientationdependent etch, sandblasting or a deep reactive-ion etch (DRIE) can beused to form the window 420 or windows (which would have vertical ratherthan sloped sides), while a KOH etch is used to form the V-grooves.Moreover, after forming the window 420 and the V-grooves, a dicing stepmay be performed for forming an alignment stop at the end of theV-grooves for aligning the optical fibers in V-grooves.

FIGS. 14A and 14B illustrate a FAU 150 with a silicon substrate 1400,according to one embodiment disclosed herein. That is, in contrast tothe FAU 150 in FIG. 4 , here, the FAU 150 includes a semiconductorsubstrate (e.g., the silicon substrate 1400) that does not include atransparent layer. For example, the silicon substrate 1400 may be lessexpensive and less complicated to form than the substrate 305 in FIG. 4. In one embodiment, the silicon substrate 1400 has a thickness greaterthan 500 microns. In another embodiment, the silicon substrate 1400 hasa thickness greater than 1000 microns.

The silicon substrate 1400 includes the alignment protrusions 415 andoptical windows 1405. For example, one optical window 1405 may bedisposed over or near the optical interface in the photonic chip, whileanother optical window 1405 is disposed over one or more of the epoxywells in the photonic chip. Although two optical windows 1405 are shown,the silicon substrate 1400 can include any number of windows 1405, e.g.,one, three, four, or more windows.

FIG. 14B illustrates a top surface 1410 of the silicon substrate 1400.The top surface 1410 is opposite a surface on the silicon substrate 1400that includes the alignment protrusions 415 and faces the photonic chip.When curing epoxy disposed between the FAU 150 and the photonic chip,radiation is emitted through the top surface 1410 and through thesilicon substrate 1400 via the windows 1405. As shown, because thetermination ends or tips of the optical fibers 155 can be seen throughthe rightmost optical window 1405, this window 1405 may be used to cureindex matching or non-index matching epoxy that is disposed between thetermination ends of the optical fibers 155 and the optical interface inthe photonic chip. The leftmost optical window 1405 may be disposed overone or more epoxy wells.

FIGS. 15A-15C illustrate forming alignment protrusions and an opticalwindow in the FAU 150 in FIGS. 14A and 14B, according to one embodimentdisclosed herein. In one embodiment, the process illustrated in FIGS.15A-15C is performed before the optical fibers and lid are attached tothe silicon substrate 1400. Further, the cross section of the siliconsubstrate 1400 in FIGS. 15A-15C corresponds to the dotted line B-Billustrated in FIG. 15A.

FIG. 15A illustrates using an orientation dependent etch to form thealignment protrusions 415. For example, a KOH etch can be used totightly control the angle of the slope of the sides of the frustumsformed by the protrusions 415. As mentioned above, these sides can formself-correcting alignment features 820 as shown in FIGS. 8-12 .

FIG. 15B illustrates using an orientation dependent etch to form aV-groove 1505 in the silicon substrate 1400. In one embodiment, thedimensions of the V-groove 1505 may be set to match the desireddimensions of an optical window. Although not required, the V-groove1505 (which serves as a portion of the optical window) may be formed inparallel with forming a plurality of V-grooves (not shown) in which aredisposed the optical fibers. Forming the V-groove 1505 in the sameetching step as forming the V-grooves for the optical fibers may reducethe time used for forming the FAU.

FIG. 15C illustrates performing a second etching step to form theoptical window 1405 which extends through the silicon substrate 1400.Rather than using an orientation dependent etch, the remaining portionof the optical window 1405 can be formed using sandblasting or DRIE.Moreover, instead of using two etching steps, in another embodiment, theoptical window 1405 is formed using a single etching step—e.g., usingsandblasting or DRIE.

Because the optical window 1405 extends through the silicon layer 1400,the window 1405 enables radiation emitted from the bottom of the siliconlayer 1400 to reach the top of the silicon layer 1400. In this manner,the window 1405 permits UV radiation to cure epoxy disposed on thephotonic chip that couples or mates with the protrusions 415.

FIGS. 16A and 16B illustrate an FAU 150 with a silicon substrate,according to one embodiment disclosed herein. FIG. 16A illustrate theFAU 150 coupled to the photonic chip 140. Although not transparent, thesilicon substrate 1600 is shown as being transmissive so that thedetails of the components beneath the silicon substrate 1600 can be seensuch as the optical fibers 155, epoxy wells, and alignment receivers. Inone embodiment, the silicon substrate 1600 a thickness greater than 500microns. In another embodiment, the silicon substrate 1600 has thicknessgreater than 1000 microns.

The silicon substrate 1600 includes an optical window 1605 formed usingan orientation dependent etchant such as KOH. FIG. 16B illustrates aside view of the optical system illustrated in FIG. 16A. In FIG. 16B,the silicon substrate 1600 is illustrated as a solid material to betterillustrate the shape of the optical window 1605. In this example, theoptical window 1605 forms a reverse frustum where the sides of theoptical windows slope towards each other while extending through thesilicon substrate 1600.

In one embodiment, the funnel created by the reverse frustum shape ofthe optical window 1605 is used to dispose epoxy onto the photonic chip140. For example, after placing the FAU 150 onto the photonic chip usingthe alignment features, epoxy may be dispensed into the optical window1605. The large base of the funnel shape of the window 1605 means thetolerance on the placement of the epoxy can much greater than usingoptical windows in some of the previous embodiments. The funnel shape(with the aid of gravity) can channel the epoxy so that the epoxy exitsthe optical window onto the desired location of the photonic chip—e.g.,between the optical interface and the termination ends of the opticalfibers or the epoxy wells. Additionally, the funnel window is etchedthrough the entire silicon FAU, so that UV light can be used to cure ortack the epoxy after dispense.

FIGS. 17A and 17B illustrate forming alignment protrusions 415 and anoptical window 1605 in the FAU 150 in FIGS. 16A and 16B, according toone embodiment disclosed herein. In one embodiment, the processillustrated in FIGS. 17A and 17B is performed before the optical fibersand lid are attached to the silicon substrate 1600. Further, the crosssection of the silicon substrate 1600 in FIGS. 17A and 17B correspondsto the dotted line C-C illustrated in FIG. 16A.

FIG. 17A illustrates using an orientation dependent etch to form thealignment protrusions 415. For example, a KOH etch can be used totightly control the angle of the slope of the sides of the frustumsformed by the protrusions 415. As mentioned above, these sides can formself-correcting alignment features 820 as shown in FIGS. 8-12 .

FIG. 17B illustrates using an orientation dependent etch to form theoptical window 1605 in the silicon substrate 1600. Specifically, thesubstrate 1600 has been flipped relative to its orientation in FIG. 17A.In one embodiment, the unmasked portion of the top of the siliconsubstrate 1600 is selected to result in the desired dimension of theoptical window at the bottom of the silicon substrate 1600. That is,given the etch angle of the orientation dependent etch and the thicknessof the silicon substrate 1600, the size of the unmasked portion at thetop of the silicon substrate 1600 can be determined that results in adesired size of the window 1605 at the bottom of the silicon substrate1600.

In one embodiment, the window 1605 is formed in parallel with forming aplurality of V-grooves (not shown) in the silicon substrate 1600 forholding and aligning the optical fibers. Forming the window 1605 in thesame etching step as forming the V-grooves for the optical fibers mayreduce the time used for forming the FAU.

FIG. 18 illustrates a molded part 1800 for a FAU 150, according to oneembodiment disclosed herein. The molded part 1800 may include a singlematerial such as a polymer or plastic which is shaped using a moldingprocess to include the alignment protrusions 415, optical windows 1805and V-grooves 1810 for aligning and holding the optical fibers. That is,instead of using a semiconductor substrate (e.g., a silicon substrate ora combination of a silicon layer and a transparent layer), the FAU 150includes the molded part 1800. The molding process can have sufficienttolerance such that the alignment shown in FIGS. 8-12 can be achieved bymating or interlock the alignment protrusions 415 to alignment receiversin the photonic chip.

The optical windows 1805 can be arranged in the molded part 1800 suchthat when the molded part 1800 is aligned with the photonic chip, atleast one of the windows 1805 is disposed over the optical interface inthe photonic chip that aligns to the termination ends of the opticalfibers disposed in the V-grooves 1810. That way, radiation can passthrough the molded part to reach index matching or non-index matchingepoxy disposed between the optical interface and the optical fibers asdiscussed above. One or more other windows 1805 in the molded part 1800can be disposed over one or more epoxy wells in the photonic chip.

In view of the foregoing, the scope of the present disclosure isdetermined by the claims that follow.

We claim:
 1. A fiber array unit (FAU), comprising: a substratecomprising: an optical window extending through a layer ofnon-transparent material, wherein the optical window is configured to bedisposed over cured adhesive and to hold excess adhesive; a plurality ofgrooves, wherein the optical window has a width that spans a majority ofthe plurality of grooves; and an alignment protrusion configured to matewith an alignment receiver; a plurality of optical fibers disposed inthe plurality of grooves, wherein the alignment protrusion is configuredto align the plurality of optical fibers with an external device whenmated with the alignment receiver; and a lid, wherein the plurality ofoptical fibers is disposed between the lid and the substrate.
 2. The FAUof claim 1, wherein the plurality of grooves and the alignmentprotrusion are formed using the non-transparent material.
 3. The FAU ofclaim 1, wherein the non-transparent material is a semiconductormaterial, wherein the substrate comprises the semiconductor materialsuch that the optical window, the alignment protrusion, and theplurality of grooves are formed from the semiconductor material, whereinthe optical window extends through a first surface and an opposingsecond surface of the semiconductor material.
 4. The FAU of claim 1,wherein the substrate comprises a transparent layer and anon-transparent layer, wherein the optical window extends through thenon-transparent layer to provide a guide for dispensing uncuredadhesive.
 5. The FAU of claim 1, wherein the substrate comprises amolded material such that the optical window, the alignment protrusion,and the plurality of grooves are formed from the molded material,wherein the optical window extends through a first surface and anopposing second surface of the molded material.
 6. The FAU of claim 1,wherein the alignment protrusion forms a frustum, wherein the alignmentprotrusion is formed using the non-transparent material.
 7. The FAU ofclaim 6, wherein at least two slanted sides of the frustum formself-correcting alignment features, wherein at least one of theself-correcting alignment features is configured to contact a side ofthe alignment receiver.
 8. The FAU of claim 7, wherein at least two ofthe self-correcting alignment features are configured to contactrespective sides of the alignment receiver.
 9. The FAU of claim 8,wherein a bottom surface of the frustum is configured to be separated bya gap from a bottom surface of the alignment receiver.
 10. The FAU ofclaim 6, wherein a bottom surface of the frustum is configured tocontact a bottom surface of the alignment receiver.
 11. The FAU of claim6, wherein a flat bottom surface of the substrate at a base of thefrustum is configured to contact a top surface of the external device,and wherein a bottom surface of the frustum is configured to bedseparated by a gap from a bottom surface of the alignment receiver. 12.An apparatus, comprising: a semiconductor substrate comprising: anoptical window extending through a layer of non-transparent material ofthe semiconductor substrate, wherein the optical window is configured tobe disposed over cured adhesive and to hold excess adhesive; a pluralityof grooves, wherein the optical window has a width that spans a majorityof the plurality of grooves; and an alignment protrusion configured tomate with an alignment receiver; a plurality of optical fibers disposedin the plurality of grooves, wherein the alignment protrusion isconfigured to align the plurality of optical fibers with an externaldevice when mated with the alignment receiver; and a lid, wherein theplurality of optical fibers is disposed between the lid and thesemiconductor substrate.
 13. The apparatus of claim 12, wherein theplurality of grooves and the alignment protrusion are formed using thenon-transparent material.
 14. The apparatus of claim 12, wherein thenon-transparent material is a semiconductor material, wherein thesemiconductor substrate comprises the semiconductor material such thatthe optical window, the alignment protrusion, and the plurality ofgrooves are formed from the semiconductor material, wherein the opticalwindow extends through a first surface and an opposing second surface ofthe semiconductor material.
 15. The apparatus of claim 12, wherein thesemiconductor substrate comprises a transparent layer of thenon-transparent material and a non-transparent layer, wherein theoptical window extends through the non-transparent layer to provide aguide for dispensing uncured adhesive.
 16. The apparatus of claim 12,wherein the semiconductor substrate comprises a molded material suchthat the optical window, the alignment protrusion, and the plurality ofgrooves are formed from the molded material, wherein the optical windowextends through a first surface and an opposing second surface of themolded material.
 17. The apparatus of claim 12, wherein the alignmentprotrusion forms a frustum, wherein the alignment protrusion is formedusing the non-transparent material.
 18. The apparatus of claim 17,wherein at least two slanted sides of the frustum form self-correctingalignment features, wherein at least one of the self-correctingalignment features is configured to contact a side of the alignmentreceiver.